Alot of people prefer the DOT4. Just be aware. DOT 3,4, and 5.1 ( NOT DOT 5) are glycol based. Where DOT 5 is silicon based. DO NOT put DOT 5 in your system. IMO mixing (even though possible) is a bad idea, do a good flush if you want to use DOT4. Some DOT 4 fluids have higher boiling points than DOT 5.1, so there are reasons why they do it. When it comes down to it, they put 5.1 in the factory for a reason, unless youre competative enough to actually feel the difference, just leave it alone
Just my 2 cents.